Existing magnetic silencing systems reduce the vulnerability of naval vessels to magnetically actuated mines and detection systems. This is accomplished through the minimization of a vessel's magnetic signature with degaussing coils or other means of generating magnetic fields. Degaussing coils are loops of wire which, when energized with the proper amount of direct electrical current, produce magnetic fields whose shape is the same as the vessel's uncompensated signature but of opposite polarity. The degaussing coils therefore cancel the undegaussed fields of the vessel.
Prior art degaussing system designs attempt to minimize the static magnetic field signature of the vessel around and along its entire length. For economic and other practical reasons, the degaussing system can not make the vessel's signature exactly zero everywhere. Therefore, a compromise is achieved by setting the current in the degaussing loops to produce the smallest signature over the largest possible area around the vessel.
A schematic diagram of a prior art degaussing system for a vessel is shown in FIG. 1 divided into three sub-systems with coils that respectively control the ship's magnetization in three orthogonal directions, ie., longitudinal sub-system 10L, vertical sub-system 10M, and athwartship sub-system 10A. Each sub-system is powered by a respective power supply 12L, 12M and 12A receiving control signals from an analog degaussing controller 14.
Degaussing controller 14 sends analog control signals to power supplies 12L, 12M and 12A. The input signals to controller 14 are the vessel's heading and a coarse indication of the geomagnetic latitude, or the earth's magnetic field measured in the vessel's three orthogonal directions with a mast mounted sensor 13. These input signals compensate the "induced" magnetization which changes with vessel motion. The permanent component of magnetization is compensated with three constant current settings whose values are determined during calibration of the degaussing system. All the degaussing loops in a sub-system (i.e., aligned in a specific direction) are connected in series. For example, the current that leaves power supply 12L enters loop 10L.sub.1, leaves loop 10L.sub.1 and enters loop 10L.sub.2, etc. The magnetic field generated by each loop is controlled by manually changing the number of conductors active in each loop. The manual adjustment of the degaussing loops and the current settings of the permanent component of magnetization are known as coil calibration.
All degaussing systems require adjustments or calibration of their operating parameters to achieve small magnetic field signatures. Calibration of the degaussing system is performed at a fleet facility called a degaussing range. At the range, the vessel sails back and forth over the top of underwater magnetic field sensors from which calibration data is obtained. Each degaussing loop is energized, one at a time, and the resultant signature measured. Then the signature of the vessel is measured with the degaussing system off. The power supply current and active turns for each loop is adjusted until both the induced and permanent components of the signature are minimized. The system is not readjusted until the vessel returns to a degaussing range. The disadvantages of existing degaussing coil systems include:
1) manual adjustment of each loop, i.e., no real-time control,
2) imprecise adjustment of each loop since turns in each loop are either wholly activated or deactivated, and
3) a compromise must be accepted in the degree of signature reduction due to the requirement that the field must be minimized over a large volume of space surrounding the vessel.